Lab Information

Lab Information
To determine the regulation of a transformer using the short circuit method.
Voltage regulation is a measure of merit for a transformer. However, it is often impractical, if not
impossible to fully load a large transformer to determine its voltage regulation. This fact presents
no problem if the short circuit tests are available, or if they can be performed. From these tests it
is possible to determine the equivalent circuit with reference to either the primary or secondary
winding. Taken from the secondary, the resistive and reactive voltage drops can be calculated
with rated current. The no-load voltage with a resistive load becomes:
Vs = rated secondary voltage
Is = rated secondary current
Rex = Equivalent circuit resistance referred to the secondary
Xex = Equivalent circuit reactance referred to the secondary
Percent voltage regulation =
Neither the no-load voltage nor the percent of regulation remain the same if the power factor is
leading or lagging. With a lagging power factor the percent of regulation and no-load voltage
increases. With leading power factor the no-load voltage regulation becomes some negative
At any lagging/leading power factor, the no load voltage is:
Es (no-load) = (cos *Vs + Is *Rex )2 + (Sin*Vs + Is Xex )2
1. One Hampden Transformer (Model T-100-3A)
2. One Hampden RLC-100 resistance-reactance load
3. One Hampden AC voltmeter
4. One Hampden AC ammeters
5. One portable voltmeter rated at least 150 volts
1. Using the information derived from the short circuit test in experiment 3, calculate the percent
load voltage regulation with reference to the secondary winding for a load drawing 0.36 A at unit
power factor. Do this by using the formula on the previous page but use the load current instead
of the rated secondary current for Is. Repeat the calculation for a 0.8 lagging power factor load
drawing 0.36A.
2. Make the connections as shown in figure 7. Adjust the load and input until approximately
0.36A flows in the secondary at rated voltage. Record the secondary and primary voltages.
3. Remove the load (remember to shut off the power before doing this). After reapplying power
record the (open circuit) secondary voltage making sure that the primary voltage is the same as
recorded in step 2.
4. Make the connections as shown in figure 8. Repeat steps 2 and 3 for a 0.8 lagging power
factor load drawing 0.36A at rated secondary voltage.
5. Calculate the percent voltage regulation for both load tests.
Prepare a written report to include:
1. A comparison of calculated values to measured valves and an explanation of the differences
between them. Include circuit diagrams and calculations.
2. An explanation of the changes in secondary voltage as the secondary load increases from a noload condition to full load while the primary voltage remains fixed.